Every kind of gas molecules has its own characteristic spectral line. So supposed that the emission spectrum of light source is overlapped with the absorption spectrum of gas, it can be designed to make the light emitted by a narrow-band light source or a laser light source pass through the gas to be detected, and then determine the concentration of the gas by measuring the intensity of the transmission light. Generally the absorption spectral linewidth of the ordinary light source is relatively wide, while the absorption spectral linewidth of some gas is very narrow. Thus the change of the light power after passing through the gas chamber is not obvious, leading to the measuring sensitivity is not very high. In contrast, the spectral lines outputted by the laser light source is relatively narrow, which is suitable for measuring a variety of gas.
Laser plays an important role in modern spectroscopy, due to its high monochromaticity (narrow spectral line), high brightness, high directivity and other unique advantages. A new laser spectroscopy develops, with an important practical application value in various research fields such as modern agriculture and environmental science, biology and medical science, physics, chemistry and materials science and astrophysics, and in industrial process monitoring.
Laser used for gas detection has an important application value in environmental detection and analysis, as well as a variety of industrial process control, etc. Each gas molecules has its characteristic spectral lines, so certain gases can be detected by using the characteristics of the laser having narrow linewidth. One of common gas detection methods by laser is to adjust or set the wavelength emitted from the laser to be consistent with the characteristic absorption spectrum line of the gas to be detected, to transmit the laser through the gas chamber, and thus to determine the concentration of the gas by measuring the attenuation of the laser after transmitting through the gas cavity. This detecting method is simple in both the principle and the structure. However, generally the ordinary light source has a wide spectral linewidth, and some of the gases have very narrow absorption lines, so the optical power does not change obviously when passing through the gas chamber, which lowers and limits measuring sensitivity. Especially, it is more difficult for detecting tiny gas concentration.
The conventional differential absorption method is achieved based on two beams in a common optical path passing through the same gas cavity to be detected. The output beam wavelength of one beam is consistent with the characteristics absorption lines of the gas to be detected. And the output beam wavelength of the adjacent beam is selected near the absorption lines of the gas to be detected, but not exactly the same with its absorption lines, to be used as a reference light in order to eliminate the instability of light intensity in the light path. However, this detection method does not eliminate the detection error caused by the instability of the wavelength of light, which can not be ignored in the practical detection. Therefore, in the prior art, the differential absorption method is improved. Commonly, the laser current and temperature is stabilized to realize a stable wavelength outputted from the laser. However such a regulation method is passive, do not strictly eliminate fluctuations of the laser, thus such an improvement do not obtain good effects.
Fiber laser is a new type laser developed rapidly in recent years. Fiber laser uses a fiber optic waveguide as a gain medium and an optical fiber grating as a feedback mirror to form an integrated optical fiber resonator, thus providing advantages such as compact structure, narrow laser linewidth, high beam quality, and a laser system with high reliability, good stability and maintenance-free, which makes a huge impact on the laser industry. Development of modern spectral composition detection and analysis system based on fiber laser will not only has great significance to the development of laser spectroscopy, but also make the fiber laser spectral analysis system more portable to be used expediently. Therefore, it is a technical problem to be solved in this field as how to apply the fiber laser in the field of gas concentration detection taking various advantages of the fiber laser, such as its compact structure, narrow linewidth of the laser output. There is a need for a gas concentration measurement method and system which not only taking advantages of fiber laser but also obtaining high sensitivity and high precision of gas detection.